1. Field of the Invention
The present invention relates to a DSRC (dedicated short-range communication) car-mounted equipment such as the one for collecting the toll used for an ETC (electronic toll collection) system in an ITS (intelligent transport system). More particularly, the invention relates to a DSRC car-mounted equipment which prevents communication error after it has entered into the communication start area as a result of improving the reception sensitivity (and transmission output) during the communication with an on-the-road equipment.
2. Prior Art
There has heretofore been known a DSRC car-mounted equipment (hereinafter also referred to simply as “car-mounted equipment”) for transmitting and receiving a variety of data through communication with an on-the-road equipment installed on a road on which a vehicle travels.
In the DSRC car-mounted equipment of this kind, in general, the reception sensitivity has been set constant irrespective of whether it is within the communication area or not. Therefore, the electric field intensity of the electromagnetic waves received from the on-the-road equipment varies depending upon the distance (position in a direction in which the vehicle is traveling) as shown in FIG. 9.
FIG. 9 is a diagram of characteristics illustrating intensities of the electric field received by the conventional DSRC car-mounted equipment, and shows a relationship between the electric field intensity for the distance from the antenna ANT of the on-the-road equipment and a predetermined level (threshold) TH corresponding to the reception sensitivity. Here, the antenna ANT of the on-the-road equipment is installed at a toll gate on a toll expressway.
In FIG. 9, the abscissa represents the position (distance in the direction in which the vehicle is traveling) of the car-mounted equipment with respect to the antenna ANT of the on-the-road equipment, and the ordinate represents the intensity of electric field received by the car-mounted equipment. The receiving area is set within about 4 meters from the antenna ANT of the on-the-road equipment.
Further, the communication start area set by the predetermined level TH (reception sensitivity) may include, depending on the environmental conditions, areas A, B where the electric field intensity so drops that the communication cannot be accomplished due to side lobes in the output from the antenna ANT of the on-the-road equipment.
As the vehicle approaches the on-the-road equipment, the communication is repeated a plural number of times between the on-the-road equipment and the car-mounted equipment as shown in FIG. 10.
FIG. 10 is a diagram illustrating the state of communication between the on-the-road equipment and the car-mounted equipment. In FIG. 10, the on-the-road equipment is repetitively transmitting the communication signal 1 to the car-mounted equipment at all times.
When the communication signal is received by the car-mounted equipment that has approached, the car-mounted equipment transmits a communication signal 2 to the on-the-road equipment.
Then, in response to the communication signal 2, the on-the-road equipment transmits a communication signal. When there is no response (communication signal 4) from the car-mounted equipment, however, the on-the-road equipment repetitively transmits the communication signal 3 (retrial transmission) until a response is received.
Here, the retrial communication operation by the on-the-road equipment is repeated about 100 to 200 times for every 2 milliseconds and, when there is quite no response from the car-mounted equipment, it is regarded that the communication is impossible (car-mounted equipment is not existing) and the communication ends.
However, due to traffic jam or some trouble, the vehicle mounting the equipment that has entered into the communication start area where the electric field intensity is not smaller than the predetermined level TH, may stay long (or may move at a very low speed) in the areas A, B where the electric field intensity drops during the plural times of retrial communication (which is a period lasting for about 0.5 seconds).
In this case, when the predetermined level TH of the car-mounted equipment remains constant as shown in FIG. 9, the communication signals 3 are not received from the on-the-road equipment, and the state where there is no response from the car-mounted equipment continues, spoiling the function of the car-mounted equipment.
According to the conventional DSRC car-mounted equipment as described above, the on-the-road equipment so judges that the communication is impossible when no communication signal 3 is received during the retrial transmission executed plural times by the on-the-road equipment while the vehicle is staying or is traveling at a low speed in the areas A, B where the electric field intensity drops, arousing such a problem that the function of the car-mounted equipment is not effectively utilized.